Automatic gain control circuit



Feb. 17, 1942. v E. w. HERoLD 2,273,107

AUTOMATIC GAIN CONTROL CIRCUIT Filed Marohso, 1939 4^ )gU//f V3 rosouncf 0F 2 E INVEN TOR. EDWARD W HERO/.D

Patented Fels. 17, 1942 AUTOMATIC GAIN CONTROL CIRCUIT 'Edward w, Herold, verona, N. J., assigner to America, a corporation of i Radio Corporation of Delaware Application March 30,` 1939, Serial N0. 264,942

4 Claims. My present invention relates to automatic gain control circuits, andmore particularly to automatic audio volume expansion or compression arrangements.

It has been customary in the reproduction of sound to compress, or contract, the volume range of the audio frequencies at the transmitting or recording end of an audio reproduction system. Such volume range compression has necessitated the employment of automatic volume expanders at the reproducing end of the system. In general, automatic expansion has been achieved in the past by rectifying a portion of the audio .input energy, and then applying the resulting direct current voltage to an audio amplifier stage so as to increase its gainwhen the audio input in operation, but are economically manufactured intensity'increased. In order to obtain sufiicient expansion, it has been necessary to employ sep- Y arately the portion of the audio input energy utilizedy for expansion. Ordinarily, at least three tubes are required; an expander, or controlled, tube, an expander voltage amplier and a rectiiier. In general, then, automatic expanders in the past have required a number of tubes, and their utilization has been expensive.

It may be stated, therefore, that it is one of the main objectsof my present invention to provide, in general, automatic gain control of a wave amplifying network, ,wherein the network com-- prises a multi-electrode tube capable of acting' simultaneously as a wave amplier, and as anv amplifier of wave energy which is to be utilized for rectification to provide a gain control voltage' for the mst-mentioned wave ampliier-section. Another important objectv of this invention is to provide in an automatic audio expansion nei:-v

work a tube having electrodes grouped in two sections; one of these sections cooperating to provide an audio amplifier, whereas the other section and assembled in audio reproducer networks.

The novel features which I believe to be characteristic of my invention are setforth in particularity in the 'appended claims; theinvention itself, however, as to'both its organization and method of operation will besijl be understood -by' reference to the following description taken in connection with the drawing in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into eiect.

In the drawing:

Fig. l shows an embodiment -of an automatic audio expander network constructed according to my invention,

Fig. 2 illustrates a modified arrangement,

Fig.' 3 illustrtaes a further modication adapted for usev in an automatic compression network,

Fig. 4 illustrates a modiiication of the invention, as shown in Fig. 3, applied to audio expansion ,controL 4 Referring now to the accompanying drawingl wherein like reference characteristics and the functions to amplify audio energy for subsequent rectiilcation whereby expansion control voltage is provided for the audio amplifier section.

Another object of this invention may be stated to reside in the provision of an audio gain control network which employs a multi-electrode tube capable of simultaneously amplifying the audio energy, and utilizing a portion of the audio input energy for trol.

Still other objects of my invention 'are to improve generally the simplicity and-felciency of expansion or compression conaudio volume expander Vor compressor circuits,

andmore especially to provide audio expander circuits which are not only. reliable and emcient ode 2, ve control electrodes.

dierent figures designate similar circuit elements, there is shown an electron discharge tube .i whichmay be of the type commercially known as a 6A? or 6A8. Such a tube comprises a cath-l ode 2 and an output electrode 3. Between these two electrodes there are disposed in successive order, in the electron stream emitted from cath- The grid, or control, electrode 4 is connected -to any desired source of audio signal energy. Such audio source may be, by way of illustration, the output circuit of the detector of a radio receiver employed in broadcast reception. However, it is to be clearly understood that the audio source may, also, be the electrical pickup of a phonograph record reproducer, or any of the preceding audio ampliiler tubes of such a reproducer network. Again, the

audio source may comprise the photo-electric pick-up of a talking mo Vit may even be the microphone of a .public adpicture system, and

dress system. In generallet it be assumed that there is impressed upon the grid electrode I. audio energy derived fro/m any desired source.

The cathode 2, input grid 4 and plate electrode.v v3 cooperate to'- lifcvide an audio amplier, and

the ampliiled s dio voltage is developed across the potentiome r resistor 5. One endof the' resistor 5 is g unded, while itsopposite end is connected to the plate electrode 3 by an audio coupling capacitor 8. 'Ihe plate electrode 3 is connected to/jany desired source of positive po- I I is adjusted by means of the variable tap 22 adapted to slide along resistor I0, the audio coupling condenser 23 connecting the tap 22 to the cathode end of resistor I3.

audio amplifier as the intensity of the audio input energy increases. Assuming that the audio input energy impressed on the control electrode 4 has previously been compressed, in anymanner well known to th'ose skilled in the art, then it bev comes essential to expand the audio volume range in order properly to secure contrast between the soft andloud sounds. For example, if music is being reproduced and the musical sounds are derived from a record employing compression in the recording, it becomes necessary to utilize expansion during audio amplification at the reproducingend of the system sokthat the proper contrast between pianissimo and fortissimo may be secured. Since these facts are well known to those skilled in theart, it is not believed necessary to provide any further explanation of the principles underlying compression and expansion of'volume range.

The gain control of the audio amplifier is provided by a portion of the audio input energy to the audio amplifier. the control energy prior to` its rectification. Hence, the second control electrode 9 is employed as a plate, or anode, electrode, and the load resistor It is arranged in circuit with anode 9.

Resistor III is connected to a source of positive To explain the functioning of the arrangement shown in Fig. 1, let it be assumed that the audio signal is applied to grid 4. The amplified audio signal voltage is developed across the output circuit of tube I. The gain of `the audio amplifier is controlled by grid I6 in a polarity sense such that the gain of the audio amplifier increases as the audio signal input intensity increases. This follows from the fact that when the audio signal voltage applied to grid 4 increases, then the audio voltage developed across resistor In increases thereby increasing the magnitude of th'e audio voltage impressed upon rectifier II. In this way -the direct current voltage across load resistor I3 increases. Since the gridn I6 is connected to the cathode, or positive,` end of resistor I3, an increase in direct current voltage developed across It is desirable to amplify y potential. It' will, therefore, b'e seen that the audio energy impressed on electrode 4 develops audio output voltage both across the output resistor 'I and the output resistor I0, both electrodes 9 and 3 functioning as audio amplifier output electrodes. Y

The audio voltage developed across output resistor I0 is impressed upon a rectifier II, and the latter may b'e of the diode type'. The cathode I2 of the diode islconnected to ground through a .path which includes the diode` load resistor I3 and the condenser I4. I connected to th'e junction of condenser; I4and load resistor I3, the junction point, additionally, being connected to a source of negative direct current potential so as to provide an initial negative bias lfor the' gain controlelectrode I6 of potential. It will be understood that the g'rids I1 and I8 providean accelerating field for the electrons between grids S` and' IBand between` resistor I3 causes the reduction of the initial negative bias on gain control grid I6. The result of this reduction of negative `bias is to increase the gain of the audio amplifier. The amount of increase in gain and thereby the amount of volume expansion may be controlled by adjustments of tap 22.

While it has been explained that tube I is of the 6A? or 6A8 type, it is desirable to have the fourth grid IIiV of a constant pitch winding. It is pointed out that in a 6A?, or GAS, type of tube the second grid S-actually' consists of the rod supports usually employed for supporting the grid Winding. With this type of construction the rod supports have a minimum or influence on the main electron stream flowing to the audio amplifier output electrode 3. It will, therefore, be appreciated that the tube I not only provides noi'- mal audio amplification, but also supplies amplincation of that portion of the audio input energy `which is to be used for rectification at tube II ode 2 provides an electron stream flowing through v`the control grid 25 to the output electrode 2B, and

audio signal energy is impressed on grid 25. 'Ihe output resistor I0 is included in circuit with the output electrode 26. In other words, cathode 2. grid 25 and plate 26 correspond respectively, in

Fig. l, toI cathode.2, grid 4 and output electrode 9.

grid It and the plate electrode 31. The gain `control Iiisconnected to the cathode end'of diode load resistor I3 by means of a resistor I9-, which functions to filterv out the .audio pulsations in the gain control voltagesfhe condenser 20 by-passes audio frequency currents Vfr or'n the grid end of resistor I9 to the cathode 2. The cathode itself is maintained at a positive potential above ground Vby meansof the usual self-biasing resistor network 2I, and, therefore, the control grid 4 is at a normal negative bias. The magnitude of the audio voltage impressed on the control rectifier The audio amplifier section of tube I' comprises the cathode 2,; the grid 4' and the output plate 3.

' The gain control electrode I6 is located between the positive screen grids, as in the case of tube I inFlg. 1.

The remainder ofthe circuit elements correspond to those shown in Fig; 1. Both signal grids 25` and 4' are connected to 4the source of audio input energy, and both of these grids are maintained at a normal negative bias by the selfbiasing network 2|; It is not believed necessary to describe the detailed construction of the combined triode-hexode type tube shown in Fig. 2; it being believed merely necessary to point out that cathode 2 yprovides a pair of independent electronstreams to a triode section and'anindependent hexode section. Ihe advantage of expansion control voltage duction of y It is desirable to design triode section is normally biased close to cut-ofi?,

yer 35 as in Fig. 3.' The grid leak 3l, however, in

' rran ement is that the control ener amplier Y a g gy ,distinction to Fig. 3 is connected to the cathode section has minimum iniluence on the function--l ing ofthe gain audio amplifierpsection. It is to be clearly understood that in either of Figs. 1 or 2 automatic compression will be secured by merely interchanging the connections to the cathode I2 and anode I5 of diode II. For example, if the anode I 5 is connected to resistor I3 in place of 'cathode I2 then with an increase of audio input energy intensity there will result. a

decrease in the gain of the audio ampliiier.

It is not essential to the operation of the present invention that independent rectiers be employed for providing the gain control voltage. In Figs. 3 and 4 are shown circuit arrangements wherein the combined triode-hexode I' may be utilized for providingthe rectification action as well as the automatic compression or expansion. In Fig. 3 there is shown an automatic compressor circuit. In this case the plate electrode 26 is connected to the same positive potential lead connected to the positive screen grids I'I and I8. The cathode Z'is connected toground by means of a pair of series resistors 30 and 3I', resistor 30 being shunted by an audio-by-pass condenser 32, and resistor 3l being shunted by an audio by-pass condenser .33.' The blocking condenser 35 connects the audio input circuit to the control grids 25 and 4' which, in turn, are connected to the junction of resistors 30 and 3I by means of grid leak 34. The gain control grid I S-is connected to the resistor 3I through the s'eries path including iilter resistor I9 and adjustable tap 22'.

The condenser ,20, connected between the grid end of resistor I9 and the cathode end of resistor 33, functions as a portion of the illter netthus permitting grid rectification. Resistor 3|' connects cathode 2 to ground, the resistor being by-passed for audio frequencies'by condenser 33.

Inthis form of -circuit the cathode 2, signal grid 25 and plate 26 provide a grid leak detector circuit. Upon a signal intensity increase. the dow of space current through cathode resistor 3l' decreases due to the well known action of the grid-leak and condenser 34-35. As a result the bias of grid. IB decreases; the gain of `audio ampliiler section 2-4r-I3-3 thereby is increased.

Expansion ofthe audio volume range results. It

the hexode section has a suiliciently 'low mu so as not to be cut 0E for-the strongest signals.' distortion eiects will be negligible.. The triode v section mu should be high, as in the case of the work I9.2Ii. It will be seen that in the arrangement of Fig. 3 no diode rectier is necessary,

-since plate' rectification occurring in the triode section of tube I is utilized. The resulting cath- -ode current change through resistor 3| is employed to increase the grid bias of grid I6 as the audio input energy increases. In other words, as

^ -the audio input voltage impressed on grids 25 and lincrea'ses there will be a greater ow of space current through resistor 3|, and, therefore, the-grid I6 will be biased increasingly in a negative polarity sense. This results in the ret e gain of the hexode section of tube I such as-s desired for automatic compression. tubeI' so that the while with the same ybi-as on grid 4' of the hexode portion vof the tube this latter portion is. operating on a substantially linear portion of its characteristic. 'I'his is readily accomplished by utilizing a higher mu, or amplification factor, for the triode section than for the grid I' to grid I'l portion of the hexode. Tubes made in this grid 25 and plate 26 from the spacing of grids 4 and I1.' If the triode section mu is not sumciently higher than the mu of grid l' to grid Il of the hexode, satisfactory operation can still be obtained by lowering the positive voltage applied to the triode section plate .26 below that of the grid II.

The circuit arrangement shown in Fig. 4 differs from that of Fig; 3 in that grid rectication is utilized in place of plate rectication in the triode section. The signal grids 25-4' are connected to g the audio signal source through the grid condensarrangement of Fig. 3. 1

While I have indicated and described several Systems for carrying my invention into eiect,

it will be apparent to one skilled in the `art that my invention is by no means limited to the particular organizations shown. and described, but

that many modications may be made without l departing from the scope of my invention, as set y forth in the appended claims.

What I claim is: v

1. In a wave transmission system, a tube comprising a cathode, an output electrode, a signal grid between the cathode and output electrode, a gain control electrode located between the signal grid and the output electrode, a second signal grid adjacent the cathode and a second output electrode; the cathode, second grid and second output electrode providing a wave detector circuit, means impressing wavesy on both signal grids, a load. impedance in the space current path o! the detector circuit connected between the cathode and a point of relatively xed alternating potential, and means responsive to voltage variations across the load impedance for audio signal input tron stream to said one output electrode, means responsive to variations in the magnitude of the electron ow to the other output electrode for adjusting the potentialof the gain control velectrode, said responsive means comprising an impedance connected in the space current paths to both'of said output electrodes and a direct current voltage connection between a point on the impedance and said gain control electrode.

V3. In a signalr amplier network, a .tube have ing a cathode, two output electrodes, a pair of input electrodes connected to a source of signal and suitably disposed to control'the electroncurrent to said output electrodes and a gain control electrode suitably disposed to control Athe electron current to a rsr| of said output electrodes,

-amplitude of said asource of direct current including. suitable biasing means applied to the electrodes of said tbe in such a manner that the direct current variaboth of said output electrodes, and means for applying only the direct current potential variations across said resistive impedance to said gain control electrode.- Y

4. In a signal amplier network, a tube having a cathode, two output electrodes, a pair of input electrodes connected to a source of signal and suitably disposed to control the electron current to said output electrodes and a gain control electrode suitably disposed to control the electron current to a first of said output electrodes, a\

10 to the rst output electrode.

E DWARD W. HEROLD. 

